We consider the entanglement between the
interior of a slab (3-dimensional strip) of comoving width \(L\)  and its exterior.
We compute the entanglement entropy through the area of the appropriate
bulk extremal surface, anchored at the expanding entangling surface on the boundary
[1]}.
Our choice of configuration is made so that explicit analytical expressions can be
derived for non-zero temperature. However, the result provides all the
necessary intuition about the dependence of the entanglement entropy on the
area of the entangling surface, as well as the enclosed volume. It also has an
advantage relative to the spherical entangling surface considered in
ref. [2]}. The coordinate transformation that we use in order to
connect the static problem with the time-dependent one remains valid for
entangling surfaces that extend beyond the cosmological horizon. This was not
the case for spherical surfaces, for which the transformation was valid up to
the horizon.
